BO-I-1: Soil moisture levels in farmland soil

The picture shows a severely dried out, cracked soil with single green stalks of grain growing out of it.Click to enlarge
Desiccated soils impair the growth of both crops and wild plants.
Source: Photograph: © Maurizio Targhetta / stock.adobe.com

2019 Monitoring Report on the German Strategy for Adaptation to Climate Change

Table of Contents

 

BO-I-1: Soil moisture levels in farmland soil

Adequate soil water reserves represent a crucial variable determining plant development. In farmland cultivation both under- and oversaturation during critical phases can adversely affect yields. In both light and heavy soils, soil water reserves available during the vegetation period have shown a significant declining trend during the past 50 years.

The line graph shows the development of soil water supply in agriculturally used soils as a percentage of the usable field capacity since 1970. The graph shows the development of soil water supply in light soils under winter cereals in May and July and in heavy soils under sugar beet in July and September. With the exception of the time series for winter cereals in July, which shows a quadratically increasing trend, the trends of the time series are significantly increasing.
BO-I-1: Soil moisture levels in farmland soil

The line graph shows the development of soil water supply in agriculturally used soils as a percentage of the usable field capacity since 1970. The graph shows the development of soil water supply in light soils under winter cereals in May and July and in heavy soils under sugar beet in July and September. With the exception of the time series for winter cereals in July, which shows a quadratically increasing trend, the trends of the time series are significantly increasing.

Source: DWD (German climate atlas - agriculture)
 

Soil water supply – potential shortages

Precipitation and temperature are important contributing factors to the process of soil formation; they have direct influence on the water regime and mineral balance of the soil. If precipitation and temperature conditions change as a function of climate change, this will have consequences for soils, no matter whether soils are used for agricultural or forestry purposes, whether urban soils or soils with near-natural vegetation are concerned. An increase in soil temperature has consequences for crop cultivation (germination and growth of plants), life in the soil (activities of countless soil organisms) and for the soil structure. Soil development processes such as weathering, decomposition and humification are accelerated. An increase in soil respiration can lead to an additional release of CO2 from the soils thus resulting in positive feedback accelerating global warming.

Subject to soil conditions it is expected that amounts of leachate are likely to diminish in summer owing to greater evaporation whereas it is expected to increase in winter owing to additional precipitation. On one hand, this results in consequences for groundwater formation. On the other, the actual amount of leachate governs the relocation of minerals such as nitrate in the soil. High precipitation in winter, especially when it falls on agricultural land with sparse vegetation cover, can result in repeated leaching of minerals. If clay-based soils dry out more in the summer months, the soil surface will harden, and water from precipitation will find it hard to seep in. In those cases repeated surface run-off will increase the risk of erosion.

For plant growth the availability of water in the soil is a crucial variable. When in the spring and summer months periods of high temperatures and low precipitation coincide with high water demand by vegetation, it can happen that the soil water available to plants is soon exhausted thus causing drought stress. For annuals which just like many agricultural plants (in particular cereal crops) flower mostly in the months of April till June when they grow fastest, restricted water supply in this phase can be critical. But also natural vegetation, for instance in wetlands, can be damaged by inadequate availability of soil water. Sandy soils, limited in their capacity to store water from winter and spring precipitation, are particularly at risk. In later phases of the vegetation period water resources are usually exhausted. Likewise, the functions of dry soils are limited also in urban areas. The summer-related heat-island effect in urban areas can become even stronger where desiccated soils lose their cooling capacity.

The DWD operates a modelling service for soil water contents. Based on prevailing meteorological conditions (data collected by the nationwide network of measuring stations) and the development stage of plants, the current evaporation of selected agricultural crops is calculated, and the calculated amount of water is then abstracted from the soil water using mathematical formulae.

The soil humidity is stated as a percentage of usable field capacity (nFK) thus identifying the water reserves in a soil which are available for use by plants. The value of usable field capacity varies with the relevant soil properties, The soil humidity % nFK enables a comparison of different soils. Where water saturation drops below a value of 50 % nFK owing to low precipitation levels, water stress is bound to set in affecting numerous plant species. Values above 100 % nFK signify that soils are saturated with water. Therefore, seepage will take place into lower soil levels.

Nationwide evaluations of mean values should be approached with caution, because soil properties and precipitation conditions differ greatly, both regionally and locally. Nevertheless, they make it possible to draw inferences for long-term development trends. For example, if you consider soil water reserves in the months of May and July for light soils with high sand contents used for winter cereal crops, it becomes clear that the nationwide mean for roughly the past 40 years shows a declining trend. As far as winter cereal is concerned, the month of May coincides exactly with the regrowth phase when water demand is particularly high, which means that adequate supplies of water are crucial for plant development. In contrast, a poor water supply in July is less serious, as this is the time when the cereal ripens. Excessively high water contents in this phase might even have adverse effects on yields, partly because excessively high water contents might limit the use of vehicles on cultivated ground.

Trends towards lower soil water reserves can also sometimes be observed with regard to heavy soils rich in clay and loam. Likewise, if you analyse the conditions prevailing in these soils used for sugarbeet (as representative for root crops), you will find in this example that July – the mid-point of the regrowth period which determines yields – is marked by declining values. September too shows a trend towards lower values. This can have adverse consequences as sugarbeet depends on adequate water supplies in order to accumulate additional biomass shortly before harvest.

Even if soil water supply is primarily dependent on precipitation conditions, farmers nonetheless have the opportunity to respond to low water contents in soils during critical development phases of plant development. One of the options is to grow less water-hungry crop species or to adapt soil cultivation methods for instance to ploughless tillage or irrigation.

 

Interfaces

BO-I-2: Rainfall erosivity

LW-I-2: Yield fluctuations

LW-R-3: Adaptation of the variety spectrum

LW-R-6: Agricultural irrigation

 

 

Objectives

Protection of soil functions (DAS, ch. 3.2.4)